As a prior art MRI imaging method, that of using multiple echoes is known.
FIG. 7 is a diagram explanatory of an example of prior art MRI imaging methods using multiple echoes. The k-space ksp is constituted of 256 (=N) views. The views are each associated with the phase from -127*gws to +128*gws (gws is the gradient amount for 1 warp step). The k-space ksp is divided into 4 (=S) blocks b1, b2, b3, and b4, with each block formed of successive 64 (=M) views. For the multiple echoes of the first order to the fourth order for the first time, data of the echoes are collected by applying a large phase gradient to the first positive side of the respective blocks b1, b2, b3, and b4. For the multiple echoes of the first order to the fourth order for the second time, data of the echoes are collected by applying a large phase gradient to the second positive side of the respective blocks b1, b2, b3, and b4. For the multiple echoes of the first order to the fourth order for the ith time, in general, data of the echoes are collected by applying a large phase gradient to the ith positive side of the respective blocks b1, b2, b3, and b4. By repeating the above operation up to the 64th time, data for 256 (=4.times.64) views of the k-space ksp are collected.
FIGS. 8A-8E are pulse sequence diagrams depicting a spin echo method for collecting data for the multiple echoes of the first order to the fourth order for the first time. The echo el of the first order provides the data of the view corresponding to the phase on the most positive side in the block b1 (phase gradient =+128*gws). The echo e2 of the second order provides the data of the view corresponding to the phase on the most positive side in the block b2 (phase gradient =+64*gws). The echo e3 of the third order provides the data of the view corresponding to the phase on the most positive side in the block b3 (phase gradient=0*gws). The echo e4 of the fourth order provides the data of the view corresponding to the phase on the most positive side in the block b4 (phase gradient =-64*gws).
In FIGS. 8A-8E, FIG. 8A shows RF pulses; FIG. 8B shows the slice axis; FIG. 8C shows the warp axis; FIG. 8D shows the read axis; and FIG. 8E shows the echoes.
In the above described prior art MRI imaging method, however, the DC view (phase gradient =0*gws) is located on the boundary line between the block b2 and the block b3 as shown in FIG. 9. Further, the low-frequency region (the hatched portion in FIG. 9) stretches over both the block b2 and the block b3. Namely, the second order echo e2 and the third order echo e3 have different signal strengths because of the T2 relaxation due to the time difference between the echo time te2 and the echo time te3, and hence, when the DC view is located on the boundary line between the block b2 and the block b3, a problem arises that a sharp difference in signal strength is produced at the DC view and the image quality is thereby deteriorated. Further, since the low-frequency region determines the contrast of the MRI image, when the data of the low-frequency region stretches over both the block b2 and the block b3 providing different contrasts, there arises a problem that the contrast of the MRI image becomes that in which the contrast in the block b2 and the contrast in the block b3 are mixed.